I agree that cultures should be changeable, but the biological traits will still affect the culture throughout. The mind is just as biological as the body, so a militaristic culture of a nonviolent species growing more naturally violent makes little more sense than a militaristic culture of a soft-bodied species developing shells
Plus if we look at real life, we see that biological features are rather immutable: there have been many human cultures and movements that sought equality, but none made the populace less likely to desire power